It is common contemporary practice to seal reciprocating or rotational shafts with a packing arrangement formed by a plurality of stacked V-shaped rings. The V-shaped rings are generally disposed in the packing chamber of a structural body in circumferentially sealing relationship to the shaft. Follower rings (also known as adaptor assemblies) are also typically employed with the follower rings and positioned on opposite axial sides of the V-shaped rings. The externally positioned follower ring is urged into axially compressive engagement with the stacked V-rings by a packing flange, with the internally positioned follower ring restrained from axial movement toward the interior of the system by the bottom of the packing chamber. Axial compression of the V-shaped ring stack tends to radially expand the rings and to assist in maintaining a sealing relationship between the V-rings and the shaft. A packing spring is also occasionally employed on the exterior side of the internally positioned follower ring to compressingly urge the internally positioned follower ring into preloaded engagement with the V-shaped ring.
Such V-rings have been formed from a wide variety of materials in the past. For example, elastomeric V-rings formed from homogenous rubbers have been used in many applications where relatively low pressures are encountered and where the fluid medial involved does not damage the rubber. Fabric V-rings coated with elastomers are often used on heavy duty equipment or when higher pressures are encountered. However, when a corrosive media is handled, the corrosive media will often attack both the rubber and the fabric rings, making them unsuitable for commercial use. Hence, under such conditions, V-shaped rings of a molded fluoronated hydrocarbon polymer, such as polytetrafluoroethylene are used. Polytetrafluoroethylene (sold under the trademark TEFLON) is inert to virtually all chemical media and is suitable for use with a very wide range of corrosive fluids. Furthermore, fluoronated hydrocarbon polymeric V-rings operate successfully through a wide range of temperatures, from approximately minus 120 degrees Fahrenheit to plus 500 degrees Fahrenheit. In addition, many of the fluoronated hydrocarbon polymers have an extremely low coefficient of friction.
Unfortunately, these fluoronated hydrocarbon polymers have a high coefficient of expansion relative to most metals. Further, when cooled after exposure to elevated temperatures, fluoronated hydrocarbon polymers may shrink to a size which is smaller than their original size. Thus, even when prior polytetrafluoroethylene packing rings are initially compressed tightly in sealing relationship about a metal shaft in a metal structural body, the sealing relationship may be lost if the system is thermally cycled.
Moreover, prior art polytetrafluoroethylene seals have been formed to the desired V-shaped configuration by compressive molding techniques. Such a process, while highly satisfactory in providing a V-shaped configuration, is not without its disadvantages. First of all, the ring must usually have a minimum thickness of approximately 1/8 inch to facilitate proper molding. As a result, when the rings are stacked in a packing arrangement, the compressive molding requirements limit the minimum stack height requirements and thus limit the number independent sealing surfaces acting on the shaft and chamber sidewalls. Additionally, compressively molded rings are permanently shaped and lack any tendency to expand out of their molded V-shape.